Vibration Analysis of an Axial Turbine Blisk with Optimized Intentional Mistuning Pattern

Abstract: Low engine order excitations (LEO) of blade integrated disks in turbocharger applications are well known to cause forced vibrations of fundamental blade modes. Hence, LEOs have to be considered as a relevant source of fatigue. Recently, efforts have been spent to mitigate the forced response by means of employing intentional mistuning patterns. This has proved to be a promising way if first the dedicated travelling wave mode of the tuned counterpart is weakly damped compared to most of the other travelling wave modes and second that sufficiently high differences are existing between inter blade phase angle-dependent maximum and minimum aerodynamic modal damping ratios. In this paper, genetic algorithms are applied to design an optimized intentional mistuning pattern for an axial turbine blisk namely to reduce the forced response of the fundamental flap mode clearly beneath that of the tuned counterpart without severely increasing the response of higher modes. Subset of nominal system mode (SNM) models are employed for that purpose in which the aeroelastic coupling is considered by means of aerodynamic influence coefficients (AIC). Two prototypes have been manufactured to prove the suitability of the approach: A first one with the optimized intentional mistuning pattern and another one with theoretically identical blades. The necessary differences of blade alone frequencies from blade to blade in case of the intentionally mistuned blisk have been accomplished by intended geometry variations. Since additional random mistuning due to the process of manufacture is unavoidable, disparities in modal properties between design intention and really manufactured blisk are expected. That is why experimental analyses are carried out in order to identify the differences between intended designs and really manufactured blisks in terms of identifying frequency based mistuning. Finally, updated structural models are used for numerical forced response analyses in order to prove the robustness of the intentional mistuning pattern with respect to additional random mistuning.